Immune dysregulation may contribute to the pathophysiologic findings seen in SCLS. A monoclonal gammopathy of unknown significance (MGUS, which is a premalignant precursor to multiple myeloma (MM), in which a clonal plasma cell population secretes monoclonal immunoglobulin (Ig, also referred to as a paraprotein) detectable in patient sera, is present in a majority of SCLS cases. Several patients with SCLS in whom MGUS evolved into frank myeloma or plasma cell leukemia experienced fewer capillary leak episodes after chemotherapy for their hematopoietic disorder. These findings suggest that the monoclonal paraprotein from the dysregulated plasma cell population may be the direct or indirect source of the pathophysiologic findings observed. We are exploring the molecular mechanisms of SCLS by examining the function of the monoclonal Ig in the development of vascular pathology in vitro. Using monoclonal Ig from SCLS patients, we will determine whether it binds to a specific cell type, target antigen (if any), and resulting effect (direct or indirect) on endothelial cells. Using 2D gel electrophoresis and mass spectrometry, we have identified proteins that appear to bind to IgG from subjects with SCLS, but not healthy controls. Confirmatory studies of potential paraprotein targets are underway. We are also characterizing the transcriptome of blood cell RNA and the proteome of SCLS serum/plasma, both pre- and post-attack, to determine whether specific biomarkers of acute symptoms can be identified. We have now evaluated nearly 40 patients at the NIH clinical center under this protocol in the last 5 years. We are the primary worldwide referral center for research on SCLS. The transient episodes of hypotensive shock and anasarca in SCLS are thought to arise from reversible microvascular barrier dysfunction. Application of episodic SCLS sera, but neither the purified immunoglobulin fraction nor sera obtained from patients during remission, to normal human microvascular endothelial cells caused vascular endothelial cadherin internalization, disruption of interendothelial junctions, actin stress fiber formation, and increased permeability in complementary functional assays. Circulating permeability factors, vascular endothelial growth factor (VEGF) and angiopoietin 2 (Ang2), were elevated in episodic SCLS sera but not in remission sera. These results suggest that humoral factors such as VEGF and Ang2 contribute to transient endothelial contraction in SCLS, suggesting a molecular mechanism for this highly lethal disorder. Current studies are aimed at examined responses of endothelial cells derived from subjects with SCLS and controls to permeability mediators. In FY 2014, we characterized a subject with atypical disease features using detailed mechanistic studies. Microvascular barrier function assays corroborated our atypical patient's clinical resistance to IVIG and suggested that his episodes were driven by acute phase cytokines IL-8 and TNF-alpha. Thus, our data suggest that SCLS may have clinically varying forms of presentation and that within the group of patients with SCLS, different cytokines may mediate capillary leak. In FY 2014, using Affymetrix Single Nucleotide Polymorphism (SNP) microarrays, we performed the first genome-wide SNP analysis of SCLS in a cohort of 12 disease subjects and 18 controls. Exome capture sequencing was performed on genomic DNA from nine of these patients as validation for the SNP-chip discoveries and de novo data generation. We identified candidate susceptibility loci for SCLS. An inbred mouse strain was identified (SJL/J), whose phenotype bears similarity to SCLS, including spontaneous vascular hypersensitivity to a variety of permeability factors including histamine (HA), serotonin, and bradykinin, important mediators of endothelial hyperpermeability in humans, and B cell neoplasms characterized by the presence of serum paraproteins. A recessive locus in SJL mice controlling systemic vascular hypersensitivity to HA (Shs) was mapped to an interval on mouse chromosome 6 syntenic with human 3p25.3, which harbors the primary genetic association with SCLS in humans. This interval contains several gene candidates, which will be analyzed in the SCLS cohort.